SRNL’s
George Wicks (left), Leung (Kit) Heung (center) and Ray Schumacher (right) with
the flame-former apparatus developed for the formation of hollow glass
microspheres.

Hollow glass microspheres have
been used for years in lightweight filler material, insulation, abrasives and
other uses. Researchers at the U.S. Department of Energy’s (DOE) Savannah River
National Laboratory (SRNL) have developed a unique variation, called
porous-wall hollow glass microspheres (PWHGMs), which offers the potential for
use in targeted drug delivery, hydrogen storage and other applications. What
makes SRNL’s patent-pending microspheres unique, and gives them the potential
to be useful in so many fields, is a network of interconnected pores in the
thin outer shells that allows the tiny “microballoons” to be filled with, hold,
and release gases and other materials.

SRNL originally developed the microspheres as a
solid-state storage method for the radioactive form of hydrogen; they have been
successfully demonstrated to store and release the gas. Subsequent work has
shown potential in other uses, including battery and medical applications.

SRNL is involved in about a half-dozen different
programs involving PWHGMs, many in collaboration with academic institutions and
industrial partners. As part of a program with Toyota, for example, SRNL
investigated filling these microspheres with special hydrogen absorbents to
develop safe hydrogen gas storage systems. A licensing agreement between SRNL
and Mo-Sci Corp., a specialty glass provider located in Rolla, Mo., will
provide SRNL with a cost-effective supply of the microspheres to continue the
research and development of additional applications.

Porosity of SRNL PWHGM walls.

Glass R&D

SRNL, the
applied research and development national laboratory located at the DOE’s
Savannah River Site (SRS) near Aiken, S.C., has long been recognized for its expertise
in the science and engineering of vitreous or glass-based systems, especially
in the waste management field. For example, the laboratory developed the
flowsheets and methods used in the SRS’ Defense Waste Processing Facility,
which incorporates and immobilizes high-level radioactive wastes into an inert
and stable glass form.

As a part of this glass experience and expertise,
SRNL has also developed a number of niches in the glass arena, one of which is
the development of porous glass systems for a variety of applications. These
porous glass systems include sol-gel systems and phase-separated glass
compositions that can be subsequently treated to produce unique types of
porosity within the glass forms. The porous glass systems can be fabricated
into a variety of forms, including coatings, plates, fibers, beads and-among
the most interesting-hollow microspheres.

Each PWHGM is about 50 microns in diameter, about
half the width of a human hair. A PWHGM’s walls, which are only 10,000 angstroms
thick (an angstrom is one-tenth of one-billionth of a meter), feature pores
that range from 100 to 1000 angstroms. This porosity results in interesting
properties, including the ability to use these channels to fill the
microballoons with special absorbents and other materials, thus providing a
contained environment for even reactive substances. Gases can enter the
microspheres and be retained on the absorbents. Also, the porosity can be
altered and controlled in various ways, and even used to filter mixed gas
streams within this system.

SRNL
glass microsphere filled with palladium, a material used for storing hydrogen;
the top of the microsphere has been removed to show the inside.

Microsphere Fabrication

The PWHGMs
are fabricated using a flame-former apparatus developed by SRNL that heats
glass powders in a hot zone formed by a controlled gas-air flame. As the glass
particles pass through the zone, the flame softens the glass and forms a
spherical particle. The glass contains a latent blowing agent that becomes
unstable as the glass is heated and forms a gas nucleus or bubble. The bubble
expands as the glass is heated and forms the hollow spheres.

These original hollow glass microspheres are
converted into PWHGMs through heat-treating to induce phase separation. This
process produces two different glass phases, one rich in silica and the other
rich in sodium borate. The sodium borate phase is an interconnected, worm-like
morphology. When it is removed by an acid leaching process, similar to the
making of commercial Vycor® glass,* it leaves
behind interconnected pores or channels that extend from the outside of the
shell to the inside.

These pores are used to fill the microspheres.
Because the glass spheres provide a protective environment, or cocoon, for
their contents, they can be used to hold reactive or flammable absorbents or
stored materials, including solids, liquids, or gases. This has the potential
to provide a safe method of handling, storing or transporting a variety of
difficult materials.

*Vycor is a product of Corning
Inc.

Future Possibilities

During the collaboration with Toyota, it was discovered that very
effective, but reactive, absorbents could be filled and subsequently protected
within the microballoons or cocoons. Bundles of nano-filaments were produced
inside as well. When chemically analyzed, these structures do not appear to be
any of the predicted known phases. This means that, in addition to new
nanostructures produced by the porosity of the microsphere walls, new phases
can possibly result. Further work is needed to clarify this interesting
finding.

SRNL puts science to work to support the
DOE and the nation in the areas of environmental management, national and
homeland security, and energy security. The management and operating contractor
for SRS and SRNL is Savannah River Nuclear Solutions, LLC. For additional
information regarding glass microspheres, contact Eric Frickey of SRNL at Bldg.
773-41A, Savannah River Site, Aiken,
SC 29808;
(803) 725-0406; e-mail eric.frickey@srnl.doe.gov; or visit srnl.doe.gov.